How to bake materials [With complex node setup] into Texture in blender 2.9?
The edges of your snow object are very large n-gons:

The vertices of the n-gons do seem to be coplanar, and the normals appear to be facing the right direction, so I’m not entirely sure why this causes any trouble. But empirically, they do seem to be the source of the issue—if I remove them, the problem goes away:


If you need the edge faces, an easy way to recreate them is to select the outermost edge loop, then extrude downwards along the Z axis:

To make the lowermost vertices coplanar, you can then press SZ0⏎ to scale to zero along the Z axis:

Now you can re-unwrap the mesh, and for baking, I recommend using UV ▸ Lightmap Pack, since that will fill the space as optimally as possible. Then re-bake:

(Note that I’ve used a small sample size here for a short baking time, which is why the result looks so noisy, but obviously you should bake at your normal resolution to get better results.)